a) Field of the Invention
The invention relates to laser scanning ophthalmoscopes which are used in clinical environments and in research to carry out fluorescence angiograms.
b) Description of the Related Art
Beyond these areas, autofluorescence has increased in importance. Formerly, there was no marketable device which enabled reliable detection of autofluorescence. The fundamental problem of autofluorescence is small signal outputs which lead to a poor signal-to-noise ratio (SNR). A known possibility for improving the SNR is by averaging over a plurality of images recorded at the same location. Since the eye of the patient is rarely stationary, movements occur between individual images which must be corrected prior to a superposition of the images. For this purpose, it is known to find prominent points in individual images, wherein displacements from one image to the other can be determined on the basis of these prominent points.
Because of the poor SNR mentioned above, individual autofluorescence images show few distinctive points, so that it is difficult to determine displacement. Known methods for finding the necessary prominent points require elaborate processing of all of the images to be superposed. This processing involves a smoothing of individual images, for example.
DE 3818084 A1 (Priority: May 27, 1987 JP P62-130832) Inventor: Akihiko Sekine
The patent describes the use of a scanning laser ophthalmoscope with two or more detectors, so that two parallel series of images can be taken into account simultaneously. An electronic arrangement is provided which allows images of one series to be superposed with images of the other series.
EP 0290566 B1 (U.S. Pat. No. 5,177,511)
A laser scanning ophthalmoscope (LSO) with a detector formed of a plurality of individual detectors is described, wherein the signal of the individual detectors is calculated to form a total signal.
It is the primary object of the invention to superpose received images with poor signal-to-noise ratios without the need to find prominent points in these images themselves.
This object is met in accordance with the invention by a laser scanning ophthalmoscope with at least a first scanner, wherein a scanning movement is generated at least in a first direction, wherein the illumination of the eye is effected alternately with different wavelengths during the scanning movement in the first direction and a first image is received for a first illumination wavelength and a second image is received for a second illumination wavelength, and a plurality of images which are received in this way are compared with one another and correction values are determined from the object displacement of images received at an illumination wavelength for the images received at the other respective illumination wavelength.
In an advantageous realization of the invention, the scanning laser ophthalmoscope uses a resonance scanner as a high-speed horizontal scanner. This resonance scanner has a swiveling mirror which carries out a sinusoidal angular oscillation considered over time. A line of a first half-image is received during the forward movement of the scanner, and a line of a second half-image is received during the return movement of the scanner. With suitably fast optical switching, it is possible to activate a laser A during the forward movement in order to receive autofluorescence signals and to activate a laser B during the return movement in order to receive reflection signals. Accordingly, it is possible to record two different images or series of images BA, BB which coincide.
Discrepancies between the images lie within the order of magnitude of a line spacing and are accordingly insignificant with respect to further processing.
The described images differ with respect to signal characteristics. While structures are barely detectable, if at all, in an individual autofluorescence image, structures can be detected in a reflection image due to the much improved signal gain.
Alternatively, rather than switching the lasers, it is possible to arrange a high-speed switchable filter (e.g., AOTF) in front of the detector and to switch this filter in such a way that a fluorescence signal or a reflection signal is included in alternate lines.
Displacements from one image to the next (for purposes of movement corrections for the superposition of autofluorescence images) can therefore be determined on the basis of low-noise, simultaneously recorded reflection images. Since the two images are received almost simultaneously, the displacements between two reflection images are identical to the displacements between two autofluorescence images. Accordingly, it is possible to correct displacements between two autofluorescence images without this displacement being determined on the basis of the autofluorescence images themselves.
Alternately, it is also possible for a reflection image to be received not only almost simultaneously, but fully simultaneously in that the reflection signal is obtained by a second detector. The second detector lies parallel to the first and is only outfitted with a different filter combination.
The invention is described more fully hereinafter with reference to the schematic drawings.